Method for insulating a borehole

ABSTRACT

A method for insulating a wellbore penetrating a subterranean region and having positioned therein an inner well casing of a smaller diameter by placing in the space between the wellbore and the inner well casing a divided, solid, closed-cell material.

United States Patent [1 1 Maxson et al.

[ Dec.3,1974

METHOD FOR INSULATING A BOREI-IOLE Inventors: Orwin G. Maxson; Gary D.

Achenbach, both of Ponca City,

Okla.

Assignee: Continental Oil Company, Ponca City, Okla.

Filed: June 28, 1973 Appl. No.: 374,439

U.S. Cl 166/292, 166/DIG. 1, 166/57, 166/315 Int. Cl E21b 33/13, E21b43/00 Field of Search 166/D1G. 1, 302, 57, 315,

Primary ExaminerStephen J. Novosad Attorney, Agent, or FirmF. LindseyScott 5 7] ABSTRACT A method for insulating a wellbore penetrating asubterranean region and having positioned therein an inner well casingof a smaller diameter by placing in the space between the wellbore andthe inner well casing a divided, solid, closed-cell material.

9 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relatesto a method for insulating boreholes penetrating subterraneanformations.

A considerable need exists for an improved method for lining boreholespenetrating subterranean formations. Substantial benefits can be derivedfrom the conservation of heat energy in steam injection wells fortertiary oil recovery, hot water injection, and production wells for theFrasch sulphur recovery process and wells for the recovery of geothermalsteam by satisfactorily insulating such wells.

A particularly acute need for an improved method for lining boreholeswith an insulating material occurs in the exploration for oil andproduction of oil in the artic regions. Major oil discoveries on thenorthern slope of Alaska and other artic areas occur in regions wherepermafrost may occur from the surface to a depth of 2,500 feet or more.The permafrost is largely made up of sand, gravel, soil, and othermaterials frozen in a matrix of solid ice. When a wellbore penetratessuch a permafrost region, the circulation of relatively warm drillingmud in the drilling operation melts the permafrost in the area of thewellbore and when the well is put on production, movement of therelatively hot oil through the permafrost region melts the permafrost inthe vicinity of the wellbore. Severe problems of enlargement of thewellbore with subsidence of material from the permafrost region havebeen encountered. This necessitates the use of extremely high strengthcasing materials and massive cementing jobs around the wellbore tomaintain the integrity of the wellbore. One method for mitigating theproblem of melting of permafrost in the region of a wellbore is toinsulate the wellbore from the surrounding permafrost region. Variousmethods have been tried. For example, casing which is sheathed withpolyurethane foam has been tried, however, beyond shallow depths, thisis entirely unsatisfactory because the hydrostatic pressure at the lowerlevels of the permafrost region collapses the cell structure of thepolyurethane foam and largely destroys its insulative properties.Attempts to form foam materi' als in the region of the wellboreemploying foaming agents asshown by Chism US Pat. No. 3,379,253 andothers are unsuccessful because the high hydrostatic pressure at thelower levels of the permafrost region prevents expansion of the gaseousfoaming agents or contracts the foams formed before injection. Likewise,inclusion of foam materials such as expanded pearlite in conventionalcements is not satisfactory because the high hydrostatic pressurecrushes such materials and significantly destroys the insulatingproperties of such cements. Therefore, while such prior art methods havesome utility for insulating boreholes in the permafrost regions near thesurface, they are not at all satisfactory at greater depths whereconsiderable hydrostatic pressure is encountered.

U.S. Pat. No. 3,722,591, issued Mar. 27, 1973, to Maxson, discloses amethod for insulating and lining a borehole in permafrost by hardeningin place a hardable, flowable composition containing microspheres. Sucha method has the disadvantage that it is difficult to remove theinsulating material when it is necessary to remove the inner casing fromsuch wells for any reason since unless the insulation is friable,removable by heat, dissolution or the like the casing is locked in placein the wellbore. In certain cases it would be economically advantageousto remove the tubing, i.e., a dry hole.

An obvious need thus exists for an improved method for insulatingboreholes penetrating permafrost regions, particularly at the lowerlevels of such formations where a considerable hydrostatic pressure isencountered and, more particularly, near the bottom of the permafrostregion which is always near its melting point.

OBJECTIONS OF THE INVENTION An object of the present invention is toprovide a method for insulating a borehole penetrating a subterraneanregion so that the insulating material is readily removed.

A further objective of the present invention is to provide a method forinsulating a borehole so that the inner casing and the like is notbonded to the insulation and is readily removable from the wellbore.

These and other objects and advantages will appear from the followingdescription of the embodiments of the invention.

SUMMARY OF THE INVENTION In one aspect, this invention discloses amethod for insulating a borehole penetrating a subterranean region andhaving positioned therein an inner casing by placing in the annularspace between the wellbore and the inner casing a divided, solid,closed-cell material.

DESCRIPTION OF PREFERRED EMBODIMENTS In the practice of the presentinvention, a wellbore penetrating subterranean formations is drilled toa desired depth. Typically an inner casing of a smaller diameter thanthe borehole is placed inside the wellbore so that fluids may be removedfrom or pumped into a desired formation. In many applications, it isdesirable that the walls of the wellbore be insulated against heattransfer to or from the liquid in the inner casing. In the practice ofthe present invention, the wellbore is insulated by placing between theouter diameter of the wellbore and the inner casing a solid, closed-cellmaterial, thereby providing insulation.

The method of the present invention will be found advantageous inwellbores used for steam injection for tertiary oil recovery, hot waterinjection in production wells for sulphur recovery processes, wells forrecovery of geothermal steam, wells penetrating permafrost zones, andthe like.

In a preferred embodiment, a wellbore is drilled to a desired depth, andthereafter an inner casing is placed inside the wellbore. The wellboreis then insulated by positioning between the wellbore and the innercasing a solid, closed-cell material. The closed cell material may beflowed into the well alone or suspended in a fluid. When suspended in afluid, the closed cell material typically comprises from about 10 toabout percent by volume of the mixture. Suitable fluids are selectedfrom the group consisting of oil, water, thixotropic fluids, andvolatile carriers. Some suitable thixotropic fluids are clay-watermixtures, asbestos-water mixtures, silica gel-water and silica gel-oilmixtures, alumina-water mixtures and the like. Some suitable volatilecarriers are hydrocarbons having a boiling point below 200C atatmospheric pressure such as propane, butane, pentane, hexane, kerosene,gas oils, liquified natural gas, heptane, octane, naphtha and the likeand water. When the closed cell material is positioned in the wellboreby the method described hereinbefore; the wellbore is insulated with aninsulating material consisting essentially of a divided solid closedcell material and atleast one gas selected from the group consisting ofthe vapors of hydrocarbons having a boiling point below about 200C,water vapor and air. In many embodiments natural gas, propane, butane orwater will be preferred because of their ready availability. As notedherein before, the closed-cell material may be introduced into the wellalone or in suspension in a fluid. A preferred method for introducingthe closed cell material is in suspension in crude oil, drilling fluid,water and the like. A further desired embodiment utilizes a thixotropicfluid. The closed cell material is suspended in a thixotropic fluidwhich is pumped into the wellbore and allowed to fill the wellbore,thereby providing an insulating layer between the wellbore and the innercasing. Such materials set and resemble solids upon standing yet arereadily rendered fluid by imparting shear into the thixotropic mixtureby agitation and the like.

It is readily seen that the closed-cell material of the presentinvention may be positioned in the wellbore by a number of methods wellknown to those skilled in the art. An important advantage of the presentinvention is that the closed-cell material is readily removed from thespace between the wellbore and the inner casing. For instance, when theclosed-cell material alone is present, it may readily be removed bypumping water into the lower portion of the wellbore and merely floatingout the closed-cell material which is typically lighter than water. Theclosed-cell material may also be re.- moved by pneumatic methods such asair injection and the like. Of course, the thixotropic suspensions willrequire that shear be imparted by pressure, agitation, and the like butare readily removed by flushing and the like. It is therefore readilyseen that by the method of the present invention, insulating materialmay be placed in position between a wellbore and an inner c'asing insuch a manner that the insulating material is readily removed whendesired such as in those instances when it is desired to remove theinner casing for repair salvage and the like.

The divided, solid, closed-cell materials of the instant inventioninclude those materials which have a particular size diameter in therange of 1 micron to 2 centimeters; over 20 volume percent void spacewherein the void space is present as closed cells; and a hydrostaticcrush strength of at least 200 psi and preferably of about 200 to about15,000 psi or more.

Presently preferred. are hollow microspheres having diameters in therange of about 1 16 to 1,000 microns which have wall thicknesses of from0.5 to 30 percent of their diameters. The walls of such microspheres canbe comprised of organic polymers such as polyurethane, surans, ceramicmaterial such as galss and the like. Specific examples of suitablematerials include certain fly ash floaters and the like. Also preferredare hollow,'siliceous microspheres having diameters in the range of to200 microns, wall thicknesses of 2-l0 microns, and a bulk density fromabout 0.5 to about 0.75 gm/cc.

The divided, solid, closed-cell materials described herein are readilyavailable from commercial sources.

While the invention has been described and discussed herein withreference to an uncased wellbore having positioned therein an innercasing the invention is by no means limited to this embodiment. Forexample the wellbore may be cased with a casing which may or may not becemented into the wellbore in which case the insulating material isdesirably placed between the inner casing and the outer casing; theinner casing may comprise tubing positioned inside an intermediatecasing and the insulating material may be placed between theintermediate casing and the outer casing, between the intermediatecasing and the tubing and the like. Many such variations andmodifications are possible within the scope of the present invention.

It is noted that the embodiments described herein before areillustrative in nature, and many variations and modifications within thescope of the present invention are possible. In fact, it is expectedthat many such variations and modifications may appear obvious anddesirable to those skilled in the art upon a review of the foregoingdescription of preferred embodiments and the following examples.

EXAMPLE 1 A 22-inch diameter borehole was drilled through a pennafrostregion which is 2,435 feet thick into the strata below for a distance ofan additional 2,000 feet. The wellbore is then cleaned of extraneousmaterial and a 4-inch inner casing is positioned in the wellbore.

A solid, closed-cell material is injected in the form of a suspension ofthe closed-cell material in propane wherein the closed-cell materialcomprises percent by volume of the suspension. The suspension is passedinto the space between the wellbore diameter and the inner casing untilthe well is substantially full, and thereafter the propane is allowed tovaporize off. An insulative liner is thus formed in situ which hasexcellent insulating properties due to the voids in the closedcellmaterial even under the high hydrostatic pressure found at the bottom ofthe borehole. This insulation could then be loosened or removed asdesired to permit retrieval of the 4-inch casing.

It is clearly illustrated that the method of the present invention iseffective in forming insulating layers between wellbore diameters andinner piping as discussed hereinbefore.

Having thus described the invention, I claim:

1. A method for insulating a wellbore penetrating a subterranean regionand having positioned therein an inner casing of a smaller diameter thanthe borehole by placing in the annular space between the wellbore andsaid inner casing an insulating material consisting essentially of adivided, solid, closed-cell material, having a particular size diameterin the range of 1 micron to 2 cm and at least 20 volume percent voidspace and at least one gas selected from the group consisting of thevapors of hydrocarbons having a boiling point below about 200C, watervapor and air.

2. The method of claim 1 wherein said closed-cell material consistsessentially of hollow microspheres having a diameter in the range of lto about 1,000 microns, a wall thickness of from about 0.5 to about 30percent of their diameter, a crush strength of about 200 to about 15,000psi and a bulk density from about 0.5 to about 0.75 g/cc.

3. The method of claim 2 wherein said subterranean region comprisespermafrost.

4. The method of claim 2 wherein said closed-cell material comprises flyash floaters.

'5. The method of claim 2 wherein said wellbore is cased and whereinsaid closed-cell material is placed between said wellbore casing andsaid inner casing.

6. The method of claim 1 wherein said insulating material is placed insaid annular space by placing in said annular space a mixture of saidclosed-cell material with at least one volatile solvent selected fromthe group consisting of hydrocarbons having a boiling point lower than200C and water and thereafter allowing said volatile solvent toevaporate.

7. A method for insulating a wellbore penetrating a subterranean regionand having positioned therein an inner casing of a smaller diameter thanthe borehole by placing in the annular space between the wellbore andsaid inner casing a mixture of a divided, solid, closedcell materialhaving a particulate size diameter in the range of 1 micron to 2 cm andat least 20 volume percent void space with a thixotropic fluid.

8. The method of claim 7 wherein said closed cell material is present insaid mixture in an amount equal to from about 10 to percent by volume ofthe mixture.

9. The method of claim 7 wherein said thixotropic fluid is selected fromthe group consisting of clay-water mixtures, asbestos-water mixtures,silica gel-water mixtures, silica gel-oil mixtures and alumina-watermixtures.

1. A METHOD FOR INSULATING A WELLBORE PENETRATING A SUBTERRENEAN REGIONAND HAVING POSITIONED THEREIN AN INNER CASING OF A SMALLER DIAMETER THANTHE BOREHOLE BY PLACING IN THE ANNULAR SPACE BETWEEN THE WELLBORE ANDSAID INNER CASING AN INSULATING MATERIAL CONSISTING ESSEENTIALLY OF ADIVIDED, SOLID, CLOSED-CELL MATERIAL, HAVING A PARTICULAR SIZE DIAMETERIN THE RANGE OF 1 MICRON TO 2 CM AND AT LEAST 20 VOLUME PERCENT VOIDSPACE AND AT LEAST ONE GAS SELECTED FROM THE GROUP CONSISTING OF THEVAPORS OF HYDROCARBONS HAVING A BOILING POINT BELOW ABOUT 200*C, WATERVAPOR AND AIR.
 2. The method of claim 1 wherein said closed-cellmaterial consists essentially of hollow microspheres having a diameterin the range of 1 to about 1,000 microns, a wall thickness of from about0.5 to about 30 percent of their diameter, a crush strength of about 200to about 15,000 psi and a bulk density from about 0.5 to about 0.75g/cc.
 3. The method of claim 2 wherein said subterranean regioncomprises permafrost.
 4. The method of claim 2 wherein said closed-cellmaterial comprises fly ash floaters.
 5. The method of claim 2 whereinsaid wellbore is cased and wherein said closed-cell material is placedbetween said wellbore casing and said inner casing.
 6. The method ofclaim 1 wherein said insulating material is placed in said annular spaceby placing in said annular space a mixture of said closed-cell materialwith at least one volatile solvent selected from the group consisting ofhydrocarbons having a boiling point lower than 200*C and water andthereafter allowing said volatile solvent to evaporate.
 7. A method forinsulating a wellbore penetrating a subterranean region and havingpositioned therein an inner casing of a smaller diameter than theborehole by placing in the annular space between the wellbore and saidinner casing a mixture of a divided, solid, closed-cell material havinga particulate size diameter in the range of 1 micron to 2 cm and atleast 20 volume percent void space with a thixotropic fluid.
 8. Themethod of claim 7 wherein said closed cell material is present in saidmixture in an amount equal to from about 10 to 80 percent by volume ofthe mixture.
 9. The method of claim 7 wherein said thixotropic fluid isselected from the group consisting of clay-water mixtures,asbestos-water mixtures, silica gel-water mixtures, silica gel-oilmixtures and alumina-water mixtures.